use mkDerivation instead of runCommand for depSpecs

[einetuer]

I had trouble building a nixos configuration that included a package built with buildGradleApplication within the gitlab ci. This fixes that.

I assume this has something to do with sandboxing being disabled? I dont really know why runCommand causes issues here, but using mkDerivation doesnt have those issues even though it is less pretty.

I tried just setting src on runCommand and also did the exact same the thing buildPhase and installPhase are doing now but that didnt seem to help either

[raphiz]

Hey, thanks for your PR! I appreciate the effort, but I don't understand why this is necessary - this sounds more like an Issue of the specific Gitlab?